The present invention generally relates to an optical communication system, and more particularly to a mechanism for switching a protection line provided in such an optical communication system when a failure has occurred.
There is known an optical communication system having optical fibers dropped in drop/insert stations and having optical fibers passing through stations without dropping them in these stations. Generally, such an optical communication system has a projection (redundant) optical fiber in order to provide for the occurrence of a failure.
Referring to FIG. 1, there is illustrated an optical communication system. The optical communication system shown in FIG. 1 has a terminal station A, and a plurality of drop/insert stations including drop/insert stations B and C. Lines L1 and L3 formed of optical fibers are dropped in (connected to) the drop/insert stations B and C. A line L2 formed of an optical fiber passes the drop/insert stations B and C without dropping therein. That is, the line L2 is not connected to the drop/insert stations B and C. It is possible to provide a protection line for each of the lines L1, L2 and L3. However, this arrangement needs a large number of lines and is thus more expensive.
For the above-mentioned reason, it is desirable to provide a protection line in common for the lines L1 and L3 dropped in the drop/insert stations B and C and line L2 passing these stations without dropping therein. In FIG. 1, a protection line includes portions PL1 and PL2 provided in common for the lines L1, L2 and L3.
In this case, two different arrangements of the protection line are available. FIG. 2A shows a first arrangement of the protection line, and FIG. 2B shows a second arrangement of the protection line. A protection line PL shown in FIG. 2A passes the station B without dropping therein, and the protection line PL shown in FIG. 2B is dropped in the station B. In the communication system shown in FIG. 2A, the terminal station A has a protection switch PSW having a hybrid circuit HYB (indicated by a dot for the sake of simplicity). A line LL1 of the terminal station A is branched into a working line WL1 and the protection line PL. A line terminating equipment LTE is provided between the protection line PL and the protection switch PSW. Similarly, a line terminating equipment LTE is provided between the working line WL1 and the protection switch PSW. A line LL2 of the terminal station A is connected to a working line WL2 via a line terminal equipment LTE.
As shown in FIG. 2A, the protection line PL passes the station B without dropping therein via two line terminal equipments LTE and two protection switches PSW of the station B. The working line WL2 passes the station B without dropping therein. The working line WL1 is dropped in the station B via a line terminating equipment LTE on the input side of the station B as well as the protection switch PSW, and goes out of the station B via the protection switch PSW and a line terminating equipment LTE on the output side thereof. An internal circuit 5 is coupled to the working line WL1, as shown in FIG. 2A.
In the optical communication system shown in FIG. 2B, the protection line PL from the terminal station A is dropped in the drop/insert station B (the protection line PL is normally open), and the protection line PL extending from the drop/insert station B is inserted therein via a hybrid circuit HYB of the protection switch PSW on the output side of the drop/insert station B. The internal circuit 5 is coupled to the working line WL1, as shown in FIG. 2B.
The protection line PL in each of the optical communication systems shown in FIGS. 2A and 2B is continuously supplied with a supervisory signal (which corresponds to some of power of a signal passing through the working line WL1). Each of the line terminating equipments LTE in the working lines WL1 and WL2 supervises the corresponding working line, and generates an alarm signal when it detects a failure in any of the working lines WL1 and WL2. Each of the line terminating equipments in the protection line PL also supervises the signal on the protection line PL, and generates an alarm signal when it does not detect the supervisory signal. On the other hand, each of the line terminating equipments LTE does not generate the alarm signal when it detects that the supervisory signal is continuously supplied.
However, the two arrangements of the protection line PL shown in FIGS. 2A and 2B have the following disadvantages, which will now be described with reference to FIGS. 3A and 3B. As shown in FIG. 3A, when a failure shown by "x" has occurred in the working line WL1 of the optical communication system shown in FIG. 2A, the protection switch PSW on the input side of the drop/insert station B is switched so that the protection line PL extending from the terminal station A is used instead of the working line WL1. Thereby, the protection switch PSW on the input side of the drop/insert station B is connected to the internal circuit 5 thereof, and the working line WL1 is grounded. As a result, the insert/drop station B can continuously perform the receiving and sending operation. However, it becomes impossible to supply the supervisory signal to the protection line PL extending from the output side of the drop/insert station B. Any drop/insert station (not shown) located on the downstream side of the drop/insert station B detects the stop of supplying the supervisory signal, and generates the alarm signal irrespective of the fact that the protection line does not have any failure. This makes the maintenance operator confused.
As shown in FIG. 3B, when a failure occurs in the working line WL2 in the optical communication system shown in FIG. 2B, the protection line PL cannot function as a through line which passes each drop/insert station without dropping therein. It will be noted that the protection line is coupled to the internal circuit 5 of the drop/insert station B and extends therefrom toward the downstream side.